The present invention relates, in general, to a memory device and, more particularly, to a nonvolatile memory device.
An Electrically Erasable Programmable Read Only Memory (EEPROM) permits stored data to be retained even if power to the memory is removed. An EEPROM cell stores data either by storing electrical charge in an electrically isolated floating gate of a field effect transistor (FET) or by storing electrical charge in a dielectric layer underlying a control gate of a FET. The stored electrical charge controls the threshold of the FET, thereby controlling the memory state of the EEPROM cell.
Conventionally, an EEPROM cell is programmed using drain side hot carrier injection to inject charge carriers either onto a floating gate or into charge trapping sites in a dielectric layer underlying a control gate. High drain and gate voltages are used to speed up the programming process. Thus, the FET in the EEPROM cell conducts a high current during programming, which is undesirable in low voltage and low power applications. Further, the EEPROM cell operates very close to breakdown during programming.
Breakdown conditions during programming can be avoided by using source side carrier injection. To program an EEPROM cell using source side hot carrier injection, a select gate is formed overlying a portion of the channel region adjacent the source region. The select gate is electrically isolated from a control gate formed adjacent the drain region. During programming, an electric field is established in the channel region so that charge carriers originating in the source region are accelerated across the channel region before being injected into a floating gate or a dielectric layer located below the control gate. The select gate controls the channel current. Thus, programming with source side hot carrier injection is more power efficient than the conventional drain side hot carrier injection and suitable for low voltage and low power applications.
Accordingly, it would be advantageous to have a nonvolatile memory device and a method for accessing the nonvolatile memory device which is power efficient and suitable for low power applications. It is also desirable for the nonvolatile memory device to be simple and inexpensive to fabricate, and silicon area efficient. In addition, it is also desirable for the accessing method to be reliable and time efficient.